In a case where AD conversion is performed in a limited area such as an area in a pixel by a signal read method designed for a solid-state imaging device, the method with the highest area efficiency is an AD conversion method of an integral type (sloped type) realized by a comparator and a digital circuit in the subsequent stage.
Non-Patent Documents 1 and 2 have been suggested as technologies for realizing AD conversion in a limited area by using an AD conversion method of an integral type. For example, Non-Patent Document 1 discloses a circuit structure in which a digital circuit in a later stage is a DRAM circuit, and a slope signal is input to a comparator more than once. When 8-bit AD conversion is performed, for example, the same slope signal is input to the comparator eight times. The operation to store a code of “0” or “1” into the DRAM circuit at the time when the output from the comparator is inverted is repeated eight times. When the comparison in the entire area is finished, the codes are read out.